The present invention relates generally to radio-signal communication and, more particularly, to systems and methods for determining the signal to interference ratio in a code-division-multiple-access (CDMA) cellular communication system.
In the past few decades, cellular communication systems have developed from test systems in a few target cities to meeting a seemingly unlimited demand internationally. A xe2x80x9ccellularxe2x80x9d communication system includes multiple communication cells (a.ka., regional zones) arranged adjacent one another to cover a larger regional area. Each cell limits the number of possible simultaneous communications to the number of channels provided in the cell. The size of the cell is defined through receivers and transmitters (a.k.a., transceivers) located within base stations that provide the communication channels through which the mobile radios communicate. Accordingly, a mobile radio communicates in a cellular system by determining the nearest base station and then establishing a radio communication link with that base station.
In providing a clear channel between the mobile radio and the selected base station, the prevention of interference from and to other radio communication links is an important concern. Generally, this concern is addressed through the use of an accurate method for determining with the nearest base station and by controlling the transmission power levels used in maintaining the communication. If the nearest base station is not accurately selected or changes without a timely update, the communication can overlap and interfere with other communications in the system. Similarly, if the transmission power used in maintaining the radio link within a given cell is not properly controlled at relatively low level, the excessive transmissions can cause intolerable levels of interference.
In some cellular communication systems, the mobile stations select the nearest base station by monitoring a control channel transmitted from each base station for its signal strength and selecting the nearest base station by comparing these channel reception levels for the best signal quality. The transmission power is controlled at minimum levels by using algorithms at the base station and/or the mobile radio and, in some systems, also by passing control information between the base station and the mobile radio during the communication.
Many recently-developed cellular systems use direct-sequence, spread-spectrum (DSSS) code-division-multiple-access (CDMA) communication, which permits more users per channel and increases channel clarity and security. In these systems, the same frequency is commonly used by a plurality of users by breaking apart the communication and transmitting using different codes. At any given frequency, the signals of other users interfere with the measurements for the signal quality of the transmitting base stations. To account for this interference, measurement methods determine the signal quality by computing the ratio of the signal reception level and the interference level, which is known as xe2x80x9cSIRxe2x80x9d or Signal to Interference Ratio. Accordingly, with the signal quality determination being directly dependent on the SIR, the operability of such cellular systems is directly dependent on the signal reception level and, therefore, directly dependent on the manner in which power levels from the base stations are controlled. For further information relating to controlling transmission power based on the received SIR, reference may be made to T. Dohi, et al: xe2x80x9cPerformance of SIR Based Power Control in the Presence of Non-uniform Traffic Distribution,xe2x80x9d 1995 Fourth IEEE International Conference on Universal Personal Communications Record, pp. 334-338, November 1995.
The industry has recognized that determining the SIR in such systems is far from obvious. For example, complexities in determining how to measure the interference include, among other factors, the communication distance between nodes and environment-dependent factors such as intra-cell orthogonality. In connection with the 3rd Generation Partnership Project Proposal 3GPP TS25.213, Version 2.3.0 (1999), various approaches have been proposed for determining the SIR in DSSS CDMA cellular communication systems. Two such approaches respectively include measuring the reception level by dedicating otherwise unused orthogonal variable spreading factor (OVSF) codes or by using designated periods in the channelization codes defined by each cellular system. OVSF codes are used by the base station transmitter to spread the signal through the bandwidth and by the receiver to despread and demodulate the received signal. While each of these proposed approaches appears feasible, their respective implementations can be relatively burdensome, complex and reduce the overall system capacity. Further, determining the SIR as a function of designated periods in the channelization codes can be disadvantageous in view of the complexity involved in calculating the interference from the whole signal. For further information relating to such proposals, reference may be made to the following papers: xe2x80x9cProposal for Downlink Interference Measurement Method, Revised,xe2x80x9d TSG-RAN Working Group 1 Meeting No. 5, TSGR1#5(99)644, submitted by Ericsson; and xe2x80x9cDownlink Interference Measurement Method Using Reserved Code,xe2x80x9d TSG-RAN Working Group 1 Meeting No. 7, TSGR1#7(99)B57, submitted by Siemens.
Accordingly, there is a need for an improved approach to signal quality measurement and SIR determination in such systems.
The present invention is directed to signal quality and SIR measurement in connection with the operation of direct-sequence-spread-spectrum (DSSS) code-division-multiple-access (CDMA) communication systems and in a manner that overcomes the above-mentioned concerns. The present invention is exemplified through a number of implementations and applications, some of which are summarized below.
Various example embodiments of the present invention are directed to overcoming the above-mentioned concerns. For DSSS CDMA cellular systems, a control channel (e.g., used for data transmission by the base station) includes a transmission gap for the transmission of data from another control channel. At a receiver (e.g., in a mobile radio), the transmission gap in the control channel corresponds to an interval in each CDMA access slot when data is not being transmitted the control channel and where the traffic of other users can be detected. This traffic is the interference factor xe2x80x9cIxe2x80x9d in the SIR and, therefore, can be used with the measured signal reception level to compute the SIR.
In one example embodiment, a signal-receiving method for use in a DSSS CDMA communication includes: locating a first control channel that is multiplexed with another channel in a received communication signal; selecting a transmission gap in the first control channel where data for other control information is being transmitted in place of the first control channel; measuring interference from other radio links in the transmission gap; and determining a signal quality for the received communication signal as a function of the measured interference.
In a more specific example embodiment, the present invention is directed to the 3GPP system that uses a primary control channel, referred to as PCCPCH (primary common control physical channel), for transmitting data from the base stations. The PCCPCH has a period of 256 chips in which data is not transmitted. In this context, a xe2x80x9cchipxe2x80x9d (or sometimes xe2x80x9cchip intervalxe2x80x9d) refers to a duration of the bit pulse in the waveform used to spread the signal in the modulation stage. The base stations multiplex the PCCPCH and another set of channels (primary and secondary SCH channels), by gapping data in the PCCPCH channel for a chip interval (e.g., 256 chips) per slot to permit transmission of data for the SCH channels. According to the present invention, this transmission gap is used by the mobile radio to measure the interference factor for the SIR.
Other example embodiments of the present invention are directed to specific methods and arrangements involving use of such a transmission gap in a control channel.